Rare-earth bonded magnets hitherto been produced by the following methods.
1. Compression molding
2. Injection molding
3. Extrusion molding
Compression molding is generally a method wherein a magnet composition comprising a magnetic powder and a thermosetting resin is filled into a mold in a press at room temperature, compressed the composition and heated to cure the resin, thereby molding a magnet. In the case of the compression molding method, since the resin content of the magnet composition is lower than that for the other molding methods, the freedom of shape in molding a magnet is smaller although the magnetic properties of the resultant magnet are superior.
Injection molding is a method wherein a magnet composition comprising a magnet power and a resin component is heat-melted to prepare a melt having sufficient fluidity which is then injected into a mold where the melt is molded into a desired shape. In the case of the injection molding, in order to impart sufficient fluidity to the magnet composition, the resin content of the magnet composition is higher than that for the compression molding, resulting in lowered magnetic properties. The freedom in molding, however, is higher than that for the compression molding.
Extrusion molding is a method wherein a magnet composition comprising a magnet powder and a resin component is heat-melted to prepare a melt having sufficient fluidity which is then formed into a shape in a die and set by cooling, thereby providing a product having a desired shape. In the extrusion, like the injection molding, the resin content needs to be high enough to impart the magnet composition to fluidity. This method has an advantage that a thin-walled and long magnet can be easily produced.
Among the above methods, injection molding and extrusion generally use a thermoplastic resin as the resin. These are disclosed in Japanese Patent Laid-Open Nos. 123702/1987, 152107/1987, 194503/1985 and 211908/1985.
However, the conventional rare-earth bonded magnet composition comprising a-rare-earth magnet powder and a thermoplastic resin, used in the prior art methods, particularly in injection molding and extrusion, has the following problems. Specifically, since the rare-earth magnet powder comprises a transition metal element, such as Fe or Co, when it is mixed and kneaded with a thermoplastic resin to prepare a composition which is then molded, the transition metal element catalytically acts on the resin component and causes an increase in molecular weight of the resin component, which results in a change in properties of the composition, such as an increase in melt viscosity. This suggests a lowering in heat stability of the rare-earth bonded magnet composition. The above phenomenon is partly described in "Journal of The Magnetics Society of Japan, vol. 16, No. 2, 135-138 (1992)," indicating that a composition comprising an Nd-Fe-B-based magnet powder and a polyamide resin, due to the influence of temperature and shearstress, undergoes changes in properties, particularly viscosity. The higher the content of the rare-earth magnet powder in the composition and the larger the specific surface area of the rare-earth magnetic powder, the higher the above tendency. The above phenomenon raises problems including that the phenomenon makes it impossible to produce a rare-earth bonded magnet composition; even though a rare-earth bonded magnet composition could be successfully produced, it cannot be stably molded due to the deterioration during molding; and it is difficult to improve the magnetic properties of the molded magnet.
For the rare-earth bonded magnet composition, the relationship between the properties of the composition and the moldability has not been fully-clarified particularly in the case of extrusion. Japanese Patent Laid-Open No. 162301/1989 discloses a method wherein the viscosity of a molding composition is specified. In this method, however, the viscosity is specified in relation to the magnetic field for alignment. Further, the resin used is a thermosetting resin, and there is no clear description on the properties, involved in the moldability, of a magnet composition using a thermoplastic resin. Furthermore, no particular attention is paid to a change in properties of the composition during moldings In actual molding, a change in properties derived from the phenomenon, as described above, occurs in the course of feed of the composition into a mold of the molding machine, which makes it impossible to conduct molding. In the case of injection molding, a sprue and a runner are generated due to the nature of the molding method and should be recycled. The change in properties of the composition renders the recycling difficult, unfavorably increasing the loss of material. This incurs an increase in cost of the rare-earth bonded magnet. In the case of the extrusion, unlike the injection molding, there is little or no need of recycling. Since, however, the operation is carried out in a continuous manner, staying of the composition in an extruder or a die often renders the molding impossible. Further, the deterioration of the composition causes a load to be applied to the machine, which often results in failure of the machine and damage to a screw and a die and a nozzle and the like of the injection molding machine.
For the magnet composition used in the extrusion, Japanese Patent Laid-Open No. 264601/1987 discloses the addition of a lubricant, Japanese Patent Laid-Open Nos. 289807/1988 and 162301/1989 disclose a magnet composition using a thermoplastic resin, and Japanese Patent Application No. 270884/1991 discloses a magnet composition having a specified viscosity. As described above, in the case of the conventional magnet composition for extrusion, the properties in a molten state and additives, such as a lubricant, are taken into consideration. However, no satisfactory consideration is given to a resin component particularly when a thermoplastic resin is used as the resin component. In the production of a rare-earth-resin bonded magnet by extrusion, in order to enhance the magnetic properties of the molded magnet, a very large amount of a magnetic powder is incorporated into the magnet composition, resulting in lowered strength, i.e., melt strength, of the magnet composition in a molten state. Therefore, in the case of the extrusion of the above composition, unlike the extrusion of a general resin, it is impossible to adopt a method wherein a resin is formed into a shape in a die which is then taken off to the outside of the die by means of a take-off device, cooled and sized outside the die to provide a final shape. For this reason, in the extrusion of a magnet composition, it is necessary to adopt a method wherein the composition is formed into a final shape in a die which, as such, is set by cooling at the forward end of the die and extruded to the outside of the die. In this method, the magnet composition, which has been set by cooling at the forward end of the die (hereinafter referred to as "cooling section"), should be extruded. This raises a problem that, when only one resin, particularly a crystalline resin, is used in the magnetic composition, the change from a molten state to a solid state is so rapid that the extrusion cannot be carried out, or the extrusion rate (molding rate) is limited by properties of the resin at a temperature around the melting point thereof.
Further, as described above, the rare-earth magnetic powder is highly active enough to deteriorate the resin component during molding, causing the resultant magnet molding to rust by oxidation when it is allowed to stand.
Among the above three methods for producing a rare-earth bonded magnet, the compression molding can produce magnets having the highest performance. Since, however, a thermosetting resin is employed as the resin, the step of heat-curing the resin must be additionally provided in the molding, so that the properties of the resin at the time of heat setting should be taken into consideration. For this reason, the resin cannot be selected based on the moldability alone, and consequently the kind and amount of the resin and the molding conditions cannot be determined from the viewpoint of the moldability alone. Furthermore, since the resin used is a thermosetting resin, defective molded body cannot be recycled.
Accordingly, the present invention provides a solution to the above problems, and an object of the present invention is to provide a high-performance rare-earth bonded magnet with high productivity. Another object of the present invention is to provide rare-earth bonded magnets having various shapes according to the applications thereof.